Recovery of biphenyl from hda polymer



United States Patent 3,401,209 RECOVERY OF BIPHENYL FROM HDA POLYMERTheodore Eugene Majewski, Midland, Mich., assignor to The Dow ChemicalCompany, Midland, Mich., a corporation of Delaware No Drawing. FiledFeb. 4, 1966, Ser. No. 525,116 2 Claims. (Cl. 260-672) ABSTRACT OF THEDISCLOSURE About 30-50% of the biphenyl present in still-bottoms fromdemethylation of toluene is recovered in 98% purity by cooling to 0-15C., washing the separated crude solid with ethanol, and fractionallydistilling.

This invention relates to a process for separating biphenyl from closelyrelated compounds. It relates particularly to a method whereby goodyields of purified biphenyl can be obtained from the still bottoms of ahydrodealkylation process.

The production of benzene by the hydrogenative dealkyl'ation ofalkylated benzenes such as toluene, xylene, and cumene is now practicedon a very large scale. This process involves the reaction of excesshydrogen with an alkylbenzene, commonly toluene, at an elevatedtemperature which is usually in the range of 600-800" C. The process maybe run in the presence of a metallic catalyst such as cobalt andmolybdenum oxides or chromium oxide or no catalyst may be used. In anysuch process, a small proportion of the hydrocarbon feed is convertedinto byproducts of higher molecular weight, particularly biphenyl. Alsoincluded among these byproducts are other polynuc'lear aromatichydrocarbons such as fluorene, naphthalene, and terphenyl as well asnon-aromatic hydrocarbons. These higher boiling byproducts are obtainedas the main constitutents of still bottoms from the distillation ofbenzene and unreacted alkylbenzene from the condensed reaction product.Ordinarily, such still bottoms contain about 30-50 percent by weight ofbiphenyl as the largest single component, lesser quantities of otherhydrocarbons as described above, and usually some unreacted alkylbenzenefeed. Although the relative proportion of these still bottoms is low ascompared to the volume of benzene produced, the actual quantity of thismaterial is large because of the scale on which the process is worked.It would, therefore, be desirable to separate any commercially valuablecomponent, particularly biphenyl, from this large volume of materialwhich would otherwise go to waste.

However, the practical separation of biphenyl in acceptable yield andpurity from this complex mixture of similar compounds presents anexceedingly difficult problem. The similar boiling points and similarsolubilities of the compounds present make either distillation orrecrystallization a generally ineffective means of separation.

It has now been found that by a combination of particular steps carriedout in a specified order, better than fifty percent of the biphenylcontained in still bottoms such as described above can be separated as apure chemical compound. Surprisingly, these same steps when carried outin a different order or like combinations of similar steps produceeither or both substantially lower recoveries and a product ofsignificantly lower quality. This novel combination comprises thefollowing steps performed in the order indicated below. It has beenfound that in a process wherein an alkylated benzene is subjected tohydrodealkylation conditions to produce a reaction mixture containingbenzene as a major constituent and biphenyl as a minor constituent andsaid mixture is fractionally distilled to obtain a distillation residueice containing at least about 30 percent by weight of biphenyl, goodrecoveries of high purity biphenyl are obtained by (1) Cooling saidresidue to 0-15 C. to cause crystallization of the major part of thebiphenyl content thereof as a crude solid,

(2) Separating the crude biphenyl from the mother liquor,

(3) Washing the crude biphenyl with an oxygen-containing water-solubleorganic solvent as further described below, and

(4) Fractionally distilling the washed crude biphenyl. By thisprocedure, better than half of the biphenyl present in the originalstill bottoms can be obtained as biphenyl of better than 98 percentpurity.

Cooling the still bottoms to at least 15 C. and preferably to 5-l0 C. isessential to obtain a practical recovery of biphenyl. These stillbottoms customarily are partially solid at room temperature, butseparation of this solid at 20-25 C., for example, provides a lowrecovery of biphenyl and the crude biphenyl thereby obtained yields aproduct no higher in quality than that from the lower temperaturecrystallization.

The washing step is essential to remove in at least substantial part animpurity or impurities which interfere with further purification bydistillation. This step, which is carried out at 030 C. and usually 'atambient temperature, requires contacting of the crude biphenyl withabout 0.1-3 ml. of solvent per gram of solid. Applicable solvents areoxygenated organic liquids capable of dissolving at least 10 percent byweight of water at 25 C. These include the lower alkanols, i.e., methyl,ethyl, propyl, isopropyl, butyl, sec-butyl, isobutyl, and tert-butylalcohols, acetone, methyl ethyl ketone, p-dioxane, and the lower alkylmonoethers of ethylene and propylene glycols. These latter includeparticularly 2- methoxyethanol and methoxypropanol, but thecorresponding ethyl, propyl, and butyl ethers can be used as well.Mixtures of these solvents are also operable and the solvent may containup to about 10 percent by weight of water. Aqueous ethanol containing05-10 percent by weight of water is preferred.

The fractional distillation step provides the final purification of theproduct. The principal impurity remaining in the crude biphenyl afterthe crystallization (or coring) and washing steps is easily andsubstantially completely eliminated by a conventional fractionaldistillation. This distillation can be carried out under atmosphericpressure, but is preferably done under reduced pressure because of thetemperatures involved. Substitution of a recrystallization for thedistillation of the crude biphenyl fails to improve the quality of theproduct significantly.

The following example illustrates a preferred embodiment of theinvention. The results thereby obtained are contrasted with the inferiorresults found by using other combinations of purification steps andcombinations of different conventional procedures.

EXAMPLE Still bottoms from the distillation of a mixture obtained byhydrodealkylating toluene at 600-800 C. over a Cr O -Al O catalystcontained 45 percent by weight of biphenyl and about 10 percent oftoluene, the remainder being largely a mixture of polynuclear aromaticand other hydrocarbons. A sample of 1605 g. of these still bottoms wascooled to 10 C., thereby causing crystallization of a yellow solid. Thecrystals were filtered from the mother liquor and were washed with two200 ml. portions of 2B absolute ethyl alcohol. The dried crystalsamounted to 576 g. Of this, 137.7 g. was fractionally distilled underreduced pressure to obtain a 91.5 g. cut boiling at 126- 129/ 14 mm.This cut had a freezing point of 682 C.

and was determined to be biphenyl of 98.6 percent purity. Experiment EThis represents a recovery of 52.3 percent of the biphenyl originallypresent in the still bottoms as a pure material.

The distillation in the above example and in the following experimentswas carried out using a Vigreux column of about 2 theoretical platesefiiciency. All of the distillations described were done undersubstantially Experiment F the same operating conditions.

A series of experiments was run on the same still bottoms using variousother combination of work-up steps.

Another portion of the biphenyl cut from Experiment D was redistilledunder reduced pressure. No biphenyl fraction having a puritysubstantially better than that of the starting out could be separated.

The procedure of example was repeated except that the washing step Wasomitted. No biphenyl fraction having purity higher than about 95 percentcould be separated.

Expenment A The procedures and results of the above example and A sampleof 1642 g, of still botto w cool d to experiments are summarized andcompared in the follow- 10 C. and the resulting yellow crystals wereseparated ing table.

TABLE Example A B C D E F Procedure Cored, 10 0., Cored, 10 C., Cored,0., Cored, 25 C., Distilled, Distilled, Cored, 10 C., washed, washed,distilled was e reeryst., redist. distilled distilled recryst. distilledwashed Percent biphenyl recovery 52. 3 34. 1 30. 1 29. 2 31. 4 Purity98.6 85. 8 98. O 98 97. 2 ca. 85 ca. 90

on a filter and washed with five portions of isopropyl 25 Iclaim:alcohol, using 50 ml. of solvent per 1 00 g. of solid for 1. In aprocess wherein an alkylated benzene is subeach wash. The washed anddried crude biphenyl amountjected to hydrodealkylation conditions toproduce a reaced to 28.0 percent of the starting still bottoms and tionmixture containing benzene as a-major constituent and analyzed 84.5percent biphenyl. A 100 g. portion of biphenyl as a minor constituentand said mixture is fracthis material was recrystallized from 58 ml. of2B abtionally distilled to obtain a distillation residue containingsolute ethyl alcohol and the crystals thereby obtained at least about 30percent by weight of biphenyl, the imwere washed with fresh alcohol. Therecrystallized prodprovement of not weighed 64.0 g. and was found to bebiphenyl of (l) cooling said residue to 0-15 C., thereby causing 85.8percent purity. Recrystallization had failed to imthe formation in saidresidue of a solid crude biprove the purity significantly. phenyl,

(2) separatin said crude biphenyl from said residue,

Expenment B (3) washing said crude biphenyl with 0.1-3 ml. per

A portion of still bottoms was cooled to 25 C. and gram f crude bi h l ft l t o ol ent from filtered to separate the crystalline solid. A partof this the group consisting of lower alkanols, acetone, methsolid wasfractionally distilled to obtain as the biphenyl l h l k lower lk l th rf eth l ne fraction a solid freezing at 66.8 C. and analyzing 98.0 l l,lower alk l mono thers, of propylene glycol, percent biphenyl. Thisfraction represented 30.1 percent -di nd ixture f said solvent with upto of the biphenyl present in the starting still bottoms. abo t 10percent by ei ht of water, and

(4) fractionally distilling the washed crude biphenyl Experiment C toobtain a purified biphenyl fraction. T ProCBdllfe 0f Expel'lment B Wasrepeated except 2. The process of claim 1 wherein the solvent is aqueousfor a Wash of the F f fi f Portions of ethanol containing up to 10percent by weight of water. methanol prior to distillation. Thedrstllled biphenyl fraction amounted to 29.2 percent of the biphenyl inthe References Ci starting still bottoms and its purity was essentiallythe UNITED STATES PATENTS same as the product of B.

3,296,323 1/1967 Myers et al 260-672 Experiment D A sample of stillbottoms was fractionally distilled to OTHER REFERENCES obtain a biphenylcut amounting to 36 percent of the Fowle and Puts! Thermalflydrodealkylatwn," C starting material and containing 81.3 percent byweight Eng-Progress 5837-40 biphenyl. This represented 65 percent of thebiphenyl in Felgelman 211-: Dealkylate Toluene BeIlZeHB," the startingstill bottoms. A portion of this biphenyl cut Y Proflesslng 44 wasrecrystallized from 2B absolute ethyl alcohol using Ismallov e1 p y FromPYFOIYSIS Product, 0.7 ml. of alcohol per g. of biphenyl cut. Therecrystal- Alfirb- Neft' K1102 44 CA 64 lized biphenyl was washed withfresh alcohol and dried1 497211- to obtain a product analyzing 97.2percent biphenyl an representing a recovery of 31.4 percent oi thebiphenyl in DELBERT GANTZ, P'lmary Exammerthe starting still bottoms, G.E. SCHMITKONS, Assistant Examiner.

